1. Field of the Invention
The present invention relates to ultrasonic modelling. More in particular, the present invention relates to a method and device for modelling the surface of an object, such as a pipe, using ultrasonic waves. The models produced with the present invention may comprise temperature models, height models, or both.
2. Background of the Invention
It is well known to use ultrasonic waves to obtain information on the surfaces of an object, for example a pole or pipe. Typically, ultrasonic pulses are transmitted towards the object, the reflected pulses are received and the travel times of the pulses are recorded. Any differences in travel times (“times-of-flight”) of the pulses are indicative of differences in the relative height of the surface and hence of the wall thickness of the object. An example of this known technique is disclosed in U.S. Pat. No. 3,930,404.
U.S. Pat. No. 5,965,818 discloses a method using ultrasonic Lamb waves to measure reduction of wall thickness due to localised corrosion at pipe supports. Two transducers are used to make a Lamb wave travel along the pipe wall in the circumferential direction. By comparing measured time-of-flight data the change in time-of-flight due to corrosion can be quantified.
However, such known methods ignore the effects that temperature may have on ultrasonic measurements. Local temperature variations may cause refraction of the ultrasonic waves, as the propagation velocity of the waves may show variations between areas having different temperatures. Refractions cause delays, that is, longer travel times, which are also indicative of height differences. Accordingly, local temperature variations may be mistaken as height differences, thus causing errors in any height model of the surface.
U.S. Pat. No. 7,286,964 discloses a method of monitoring structural health conditions of objects by using acoustic waves, for example Lamb waves, and producing a tomographic image. This known method also involves determining an ambient temperature adjustment parameter. The adjustment is applied to the whole surface being monitored and is therefore not local. As a consequence, refraction due to local temperature variations is not compensated and measurement errors are likely to occur.
International Patent Application WO 2004/099764 discloses a method for determining structural features in a pipe, duct, container or other objects by using acoustic signals. The effects of temperature on the method are not mentioned in said document.
British Patent Application GB 2 300 717 discloses a method of modelling the temperature in segments of a pipeline. The liquid temperature and the ambient temperature are measured directly at several stations. Then the influence of kinetic energy, radiation and conduction are used to calculate the temperature in pipeline sections. The calculated section temperature is used to determine the net expansion and contraction of both the liquid and the pipeline. The spatial resolution of this known method is limited by the number of measuring stations and pipeline sections. The method will not be able to provide a circumferential temperature distribution of a pipeline section, nor a longitudinal temperature distribution having a high resolution.